1. Field of the Invention
The present invention concerns wireless communications systems (e.g., cellular wireless systems) that utilize multiple transmit antennas, and is particularly directed to the problem of optimizing the signal weights applied to such multiple transmit antennas so as to increase total signal power at the receiver antenna.
2. Description of the Related Art
The capacity of a wireless radio frequency (RF) link or of an entire wireless network can be improved by using multiple transmit antennas, in the form of an antenna array. Generally speaking, the goal is for the transmission to be adapted such that the magnitude and phase of the sinusoidal radio carriers emitted from each transmit antenna add coherently at the receiver antenna (or multiple receiver antennas), hence providing a larger receiver power for a given total transmit power (as summed over all transmit antennas). This reduces the contribution of the transmission to the interference experienced by other receivers in the network, and therefore increases the system capacity. The adjustment of the magnitude and phase of the sinusoidal transmissions generally is represented as the application of a complex baseband weight vector, where the magnitude and phase of each complex element of the weight vector represents the magnitude and phase of the corresponding antenna transmission. Accordingly, such magnitude and phase adjustments are referred to herein as a set of complex weighting values or, alternately, as a weighting vector.
Various conventional techniques have been proposed for generating such weight vectors. However, such conventional techniques have a variety of problems, such as: being overly complex, requiring too much overhead in the form of feedback from the receiver, requiring too much processing at the receiver (which is a particular problem where the receiver is a mobile station in a wireless network), requiring too much transmit power to be dedicated to probing for better baseband weight vectors, and/or simply providing baseband weight vectors that are too far from the optimal solution. What is needed, therefore, are techniques that provide efficient antenna weighting vectors while still addressing the foregoing problems.